"GetSizeForItem" for a UICollectionView in Xamarin with UICollectionViewFlowLayout does not get called

Question

I have a UIViewController that has a UICollectionView added as a subview in the viewDidLoad. I am using a UICollectionViewFlowLayout and have created a custom MvxCollectionViewSource to manage the cells.

I provide the cell information in the constructor of the layout object which does not make it flexible for device and orientation changes.

UICollectionViewLayout layout = new UICollectionViewFlowLayout () {ItemSize = new SizeF (354, 150),
            ScrollDirection = UICollectionViewScrollDirection.Vertical
        };

Allocating my UICollectionView in the ViewDidLoad method.

CollectionView = new UICollectionView (this.View.Bounds, layout);
        CollectionView.Delegate = new CustomViewDelegate();
        CollectionView.RegisterClassForCell (typeof(ProjectCollectionCell), ProjectCollectionCell.Key);
        CollectionView.Source = new ProjectCollectionSource (CollectionView, this,ProjectCollectionCell.Key);
        CollectionView.BackgroundColor = UIColor.Clear;
        //          CollectionView.RegisterClassForSupplementaryView (typeof(Header), UICollectionElementKindSection.Header, headerId);
        CollectionView.AllowsMultipleSelection = true;
        this.View.AddSubview (CollectionView);

Now the ProjectCollectionSource is my custom source and have created a UICollectionViewDelegateFlowLayout as mentioned here.

The UICollectionViewDelegateFlowLayout is as below but it never gets called.

class CustomViewDelegate: UICollectionViewDelegateFlowLayout
    {

        public override CoreGraphics.CGSize GetSizeForItem (UICollectionView collectionView, UICollectionViewLayout layout, NSIndexPath indexPath)
        {
            return new CoreGraphics.CGSize (100, 100);
        }
    }